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    <title>Document</title>
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    <script>
        /* 乘法运算中，NaN*任何数字都是NaN
            除法也是一样 */
    var a=1;
    var b="20";
    var c="true"
        console.log(a*b);//200
        //Number(a)*Number(b)  10>20
        console.log(a*c)//NaN
        //Number(a)*Number(c)  10*NaN
        console.log(b*c)//NaN
        //Number(b)*Number(c)  20*NaN
        var d=1;
        d+++d;
        var f=2;
        f---f;
        console.log(d);
        console.log(f);
    </script>
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